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Instructions Polycom, Modèle OMNISMART OMNIVS500

Fabricant : Polycom
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An AEC solution that is poorly designed or inappropriate for the location will not provide these benefits and can even degrade audio quality significantly. There are many AEC solutions available, ranging in price from several dollars to several thousands of dollars. There is also a broad range in quality and performance. This paper outlines some guidelines for determining the performance needed in a given location, judging the performance of an AEC solution based on specifications and listening tests, and ultimately finding the best AEC solution for any application. Presented first, however, are guidelines for verifying that an acoustic echo canceller is necessary in an application. Why AEC? Acoustic echo cancellation is useful in any hands-free telecommunications situation involving two or more locations. Acoustic echo is most noticeable (and annoying) when delay is present in the transmission path. This would happen primarily in long distance circuits, or systems utilizing speech compression (such as in videoconferencing or digital cellular phones). Even though this echo might not be as annoying when there is no delay (such as with short links between conference rooms in the same building or distance learning over fiber-optic cable), room acoustics will still affect the sound and may hamper communication. Also, howling can occur if the microphone is positioned too close to the speaker whether or not there is transmission delay, and is eliminated by most acoustic echo cancellers. Acoustic echo cancellers can be used in both narrow band (3.5 kHz) and wide-band (7 kHz) conferencing systems. Narrow band applications include teleconferencing and low bit-rate video conferencing. Wide- band applications include high quality teleconferencing and videoconferencing, as well as distance learning. Users of wide-band conferencing systems should be particularly interested in using acoustic echo cancellation, as it would allow them to make the most out of the additional audio capabilities of their systems. The primary beneficiaries of an echo canceller are the people at the far (or remote) end of the transmission path. The near (or local) echo canceller prevents the echo of the remote peoples’ voices from being returned (i.e. echoed) to them through the audio system. People speaking on the same (local) end as the AEC should not notice the AEC if it is doing its job properly. While the people on the far end receive the benefit of better audio quality, it also enables the conversation to flow more smoothly, benefiting both parties. AECNonlinearProcessingMicSpeakerEchoesNear speechFar speechFar speechNear speech + echoes + distortionFar EndNear EndXMTRCVDistortion Figure 1: Illustration of the effects of AEC operation and room acoustics on the transmitted speech. The far speech that travels through the receive path is not modified as it passes through the AEC. In an echo canceller that is poorly designed, there may be residual echoes as well as distortion added to the near speech signal (these effects are described in detail later). This degrades the speech that is transmitted, so that the poor audio quality is noticed on the far end. Why not just use a speakerphone? Speakerphones are half-duplex, which means that only one person can talk at a time. The speakerphone determines which side is active (or louder) by comparing the signal levels on both sides. It turns off the other side until the louder side is finished. Once one side has “captured” the circuit, most speakerphones do not permit any sort of interruption. This inhibits the natural flow of conversation, especially if one party is long-winded. Acoustic Echo Cancellation vs. Line Echo Cancellation With acoustic echo cancellation, the echo path is complex and can vary continuously as people move around the room. Acoustic echo cancellation and line echo cancellation both address similar problems, and are often based on the same technology. However, a line echo canceller generally cannot replace an acoustic echo canceller, because acoustic echo cancellation is a more difficult problem. With line echo cancellation there are generally only one or two reflections from telephone hybrids or impedance mismatches in the telephone line. These echoes are usually delayed by less than 32 ms, and do not change very frequently, if at all. With acoustic echo cancellation, the echo path is very complex (dozens or hundreds of reflections), lasts 100-200 ms, and can vary continuously during a conversation as people move around the room. Acoustic echo cancellers are therefore much more complicated devices. While line echo cancellers may have smaller price tags, they can’t perform under the conditions that acoustic echo cancellers can handle. Steps to Choosing An AEC Now that the need for an acoustic echo canceller is recognized, the best AEC solution for the application isdetermined based on the following four step selection process. • Find AEC products with the features and form factor...